Atrialcontractionisnotessentialforventricularfilling,ascanbeobservedinpatientswithatrialfibrillationorcompleteheartblock.Inatrialfibrillation,theatrialmyofiberscontractinacontinuous,uncoordinatedmannerandthereforecannotpumpbloodintotheventricles.Incompleteheartblock,theatriaandventriclesbeatindependentlyofeachother.However,ventricularfillingmaybenormalinpatientswiththesetwoarrhythmias.Incertaindiseasestates,theAVvalvesmaybemarkedlynarrowed(stenotic).Insuchconditions,atrialcontractionplaysamuchmoreimportantroleinventricularfillingthanitdoesinanormalheart.
The changes in left ventricular pressure and volume throughout the cardiac cycle are summarized in
Fig. 16.43 . Diastolic filling starts when the mitral valve opens (point A in
Fig. 16.43 ), and it terminates when the mitral valve closes (point C). The initial decrease in left ventricular pressure (from points A to B), despite the rapid inflow of blood from the left atrium, is attributed to progressive ventricular relaxation and distensibility. During the remainder of diastole (from points B to C), the increase in ventricular pressure reflects ventricular filling and changes in the passive elastic characteristics of the ventricle. Note that only a small increase in pressure accompanies the substantial increase in ventricular volume during diastole (from points B to C).
•Fig. 16.43 Schematizedpressure-volumeloopoftheleftventricleforasinglecardiaccycle.
The small pressure increase reflects the compliance of the left ventricle during diastole. The small increase in pressure just before the mitral valve closes (to the left of point C) is caused by the contribution of atrial contraction to ventricular filling. With isovolumic contraction (from points C to D), pressure rises steeply, but ventricular volume does not change because the mitral and aortic valves are both closed. When the aortic valve opens (point D), and during the first (rapid) phase of ejection (from points D to E), the large reduction in volume is associated with a steady increase in ventricular pressure. This reduction in volume is followed by reduced ejection (from points E to F) and a small decrease in ventricular pressure. Closure of the aortic valve (point F) is followed by isovolumic relaxation (from points F to A), which is characterized by a sharp drop in pressure. Ventricular volume does not change during the interval between closing of the aortic valve and opening of the mitral valve (from points F to A) because both the mitral and aortic valves are closed. The mitral valve opens (point A) to complete one cardiac cycle.
Several key cardiovascular system parameters are evident on a left ventricular pressure-volume loop (P-V loop), or may be calculated from it. End-diastolic volume is obtained at mitral valve closure (point C in
Fig. 16.43 ) and the end-systolic volume at mitral valve opening (point A). The stroke volume is then apparent as the “width” of the P-V loop and is calculated as follows:
The preload of the left ventricle, considered here as left ventricular end-diastolic pressure, is the pressure coordinate when the mitral valve closes (point C in